Actor-served files: /actor/<id>/fs/

File operations under /actor/<id>/fs/ become messages from `fs` to <id>
(fs_read, fs_write, fs_mkdir, fs_remove, fs_rename) with a 10 s timeout.
vfs.rs wraps the wasi:filesystem implementation of wasmtime-wasi.

PROTOCOL.md

@@ -78,6 +78,33 @@ so that readers never see half of it, write a temporary file and `rename` it
 Send large or binary data as a path under `/actor/<id>/public/`, not inside a
 message.
 
+### Actor-served files: `/actor/<id>/fs/`
+
+Files under `/actor/<id>/fs/` are not on disk: the actor `<id>` serves them.
+Every actor can read and write them. The host turns each file operation into a
+message from `fs` to `<id>` and waits up to 10 seconds for the reply. `path`
+is relative to `fs/` and starts with `/` (the folder itself is `/`).
+
+| Request | Reply |
+| --- | --- |
+| `{"type": "fs_read", "path"}` | `{"type": "fs_file", "text"}` (or `"base64"` instead of `"text"`), or `{"type": "fs_dir", "entries": [{"name", "type": "file" \| "dir"}]}` |
+| `{"type": "fs_write", "path", "text"}` (or `"base64"` for non-UTF-8 data) | `{"type": "fs_done"}` |
+| `{"type": "fs_mkdir", "path"}`, `{"type": "fs_remove", "path"}` | `{"type": "fs_done"}` |
+| `{"type": "fs_rename", "path", "to"}` | `{"type": "fs_done"}` |
+
+Any request can be answered with `{"type": "fs_error", "errno": "no-entry"}`.
+`errno` is a `wasi:filesystem` error code: `no-entry`, `exist`,
+`not-directory`, `is-directory`, `not-empty`, `access`, `not-permitted`,
+`invalid`, `unsupported`, `busy`, or `io` (also for anything else).
+
+- Opening a file sends `fs_read` once. Reads come from that copy.
+- After a change, closing (or syncing) the file sends the whole file in one
+  `fs_write`.
+- No reply within 10 seconds, or an actor that does not exist, makes the
+  operation fail (`io`, or `no-entry`). An actor that opens its own `fs/`
+  waits for itself and fails after the timeout.
+- `fs` is reserved: no actor can have that ID.
+
 ## The broker
 
 Requests to `broker`. The answer is a message whose `reply_to` is the ID of